Results 1 - 10 of 1363
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[en] Results of studying stripe quantum cascade lasers emitting at room temperature in the spectral range of 4.8 µm are presented. Power characteristics and turn-on dynamics of the lasers upon pulse pumping are studied. The performed investigations demonstrate the presence of a significant heating of the active region during the pump pulse.
[en] Ionized gas in low-temperature conditions, are in general characterized by the presence of molecular species, which can strongly affect the system properties and its evolution. In particular, when non-equilibrium conditions exists, the interactions occurring among the particles at a microscopic level, give rise to a complex collisional physics, largely dominated by molecules in different internal quantum states which act as different chemical species. So the modeling of these systems requires the characterization of a plethora of collisional processes, involving molecules, where exchanges of ro-vibronic energies, dissociation, ionization, reactive processes etc., may occur. In this frame, the availability of large sets of cross section data becomes a crucial prerequisite for a realistic simulation of plasma system.
[en] Nanostructure with an interior nanogap has received much attention in surface-enhanced Raman scattering (SERS). However, controllable synthesis of nanostructure with ultrasmall nanogap and innovative morphology still remains a challenge. Herein, we present a facile seed-mediated route to integrate uniform nanogap in novel Ag nano coix seeds and locate Raman molecules in the nanogap at room temperature. After 20-nm Ag nanoparticles (NPs) modified by Raman ligand 2-naphthalenethiol and coated by polymer shells as cores were obtained, outside Ag shells were formed by in situ reduction on the polymer surface. The SERS properties of these resulting Ag nano coix seeds were systematically explored. More importantly, the novel SERS active substrate exhibited ultrahigh homogeneity, reproducibility, stability, and even a reliable quantitative SERS analysis mediated by internal standard molecules. .
[en] This study investigates the risks of non-conservative piping design according to ASME B31.1 for high-temperature piping subjected to long-term operation at high temperature in a creep regime based on a sensitivity analysis of the hold time. Design evaluations of high-temperature piping were conducted over a range of hold times in the creep regime according to ASME B31.1, which implicitly considers the creep effects, and the French high-temperature design code of the RCC-MRx, which explicitly considers the creep effects. Conservatisms were quantified among the codes in terms of the hold times. In the case of B31.1, the design evaluation results do not change depending on the hold time at high temperature, whereas in the case of RCC-MRx, they do. It was shown that the design limits of RCC-MRx were exceeded when the hold time exceeded certain values, whereas those of B31.1 were satisfied regardless of the hold times. Thus, the design evaluations according to B31.1 did not consistently yield conservative results and might lead to non-conservative results in the case of long-term operations in the creep range.
[en] Nanodroplet formation is a critical process in the development of 3D nano-inkjet printing. We show that many-body dissipative particle dynamics (MDPD) can be used to predict nanodroplet formation in nanosized nozzles with good accuracy. A conversion methodology is also introduced to overcome the problem of large coarse-graining factor, which results in unphysical results when the simulation is scaled up to real units. Using our MDPD model and the new conversion methodology, insights into possible trends of physical quantities in nanodroplet formation of polymeric ultraviolet ink can be gained. It was found that higher temperature and applied pressure reduce droplet break-up time. In addition, higher temperature increases the droplets’ diameter while higher effective pressure reduces it. These findings suggest that the physical environment can be tuned to achieve the desired droplet properties for 3D nano-inkjet printing. Due to the technical challenges that impedes experimental testing, this work demonstrates that MDPD provides a low-cost alternative to study nanodroplet formation in 3D nano-inkjet printing. (paper)
[en] Controlling and measuring the temperature in different devices and platforms that operate in the quantum regime is, without any doubt, essential for any potential application. In this review, we report the most recent theoretical developments dealing with accurate estimation of very low temperatures in quantum systems. Together with the emerging experimental techniques and developments of measurement protocols, the theory of quantum thermometry will decisively impinge and shape the forthcoming quantum technologies. While current quantum thermometric methods differ greatly depending on the experimental platform, the achievable precision, and the temperature range of interest, the theory of quantum thermometry is built under a unifying framework at the crossroads of quantum metrology, open quantum systems, and quantum many-body physics. At a fundamental level, theoretical quantum thermometry is concerned with finding the ultimate bounds and scaling laws that limit the precision of temperature estimation for systems in and out of thermal equilibrium. At a more practical level, it provides tools to formulate precise, yet feasible, thermometric protocols for relevant experimental architectures. Last but not least, the theory of quantum thermometry examines genuine quantum features, like entanglement and coherence, for their exploitation in enhanced-resolution thermometry. (topical review)
[en] TmMgGaO is a quasi-two-dimensional triangular spin system with Ising anisotropy. As a sister compound of YbMgGaO, which recently was identified as a spin liquid material, it turned out to show a different magnetic ground state at low temperatures. In terms of an almost absent zero-point entropy and distinct anomalies in ac susceptibility a variety of exotic magnetically ordered multipolar phases is observed at very low temperatures. In our work, we present latest results of ac-susceptibility and vector-magnetometry experiments performed on single-crystalline TmMgGaO at lowest temperatures and up to high magnetic fields.
[en] Study of microstructure evolution in the form of grain growth in polycrystalline materials has been an important goal for material scientists as it drastically affects physical and mechanical properties. Specifically, nanocrystalline materials, which are known for their superior mechanical properties, are highly susceptible to grain growth even at low temperatures and stresses. Various experiments and simulations carried out on nanocrystalline materials indicate that the microstructure evolution in these materials takes place due to grain boundary migration and grain rotation. Therefore, migration of grain boundaries and grain rotation-induced grain coalescence contribute in increasing the average grain size in the microstructure. In order to simulate microstructure evolution in polycrystalline materials, the multi-order parameter phase-field model is a popular approach and is widely used for studying evolution purely due to the grain boundary migration. In this work, we present a multi-order parameter phase-field model capable of capturing microstructure evolution due to grain boundary migration and grain rotation-induced grain coalescence. The model couples constitutive equation of viscous sliding-induced grain rotation and the classical phase-field model for curvature-driven grain boundary migration. This paper covers various topological and statistical aspects of the microstructure evolved in the presence of both these growth mechanisms in great detail.
[en] This study investigates the combined effect of the strain rate and temperature on the compressive properties of hybrid fiber ultra high toughness cementitious composites (UHTCCs) using a Split Hopkinson pressure bar. Specimens were first heated to different exposure temperatures, e.g. ambient temperature, 200, 400, 500, 600 and 800 °C, and subsequently, cooled to ambient temperature. Thereafter, the specimens were tested at four different strain rates. The test results show that the dynamic compressive strength of the UHTCC is enhanced at a temperature of 200 °C, and subsequently, decreases with the increase in exposure temperature. The strain rate sensitivity of UHTCC is largely enhanced with the increase in exposure temperature. The possible mechanism of this phenomenon was discussed based on the high-speed photography of the crack propagation process on the surface of the specimens and microscopic observation of fibers condition on their fracture surfaces. Moreover, an empirical relationship is established to express the dynamic strength enhancement of fire-damaged UHTCC as a function of strain rate.
Abstract—This paper presents a detailed experimental study of phase formation processes in a mechanically activated Ti + Al powder mixture. High-temperature synthesis has been performed in thermal explosion mode using induction heating of the mixture. We present the first evidence that, during a continuous transition from rapid heating to high-temperature annealing, the composition of the synthesis products depends on the secondary structuring time. Early stages of annealing involve structural relaxation processes, which make the phase composition more uniform and lead to the formation of an essentially single-phase TiAl compound. In later stages, the system undergoes a transition to thermodynamic equilibrium, which is accompanied by the formation of compounds that are in equilibrium at the annealing temperature.